Most of the information available on the mechanical properties of cardiac muscle have come from the study of isolated papillary muscles of trabeculae. These preparations are necessarily damaged by the attachment to the apparatus. We have recently developed a technique to eliminate the effects of the damaged ends by measuring a servocontrolling a central undamaged segment of the muscle marked by 100 micrometer diameter pins. Papillary muscles will be studied using the new technique to (1) separate crossbridge elasticity from passive series elasticity in cardiac muscle, (2) determine whether a change in inotropic state changes only the number of attached crossbridges during contraction or the properties of the crossbridges, (3) determine the influence of velocity of shortening on the number of crossbridges and whether inotropic agents usch as neurotransmitters influence this relationship, (4) determine the systolic force-velocity-length-time surface for cardiac muscle and quantitate the changes induces by neurotransmitters or other agents which change inotropic state, and (5) characterize the isometric force-length relationship of cardiac muscle so as to separate pure geometric factors from length-dependent activation and two separate length dependent activation into fast and slow components. We feel that the research proposed will elucidate some of the physical properties of cardiac muscle and further our understanding of the mechanisms of cardiac muscle contraction and of endogenous (largely neurotransmitters e.g. norepinephrine and acetyl choline) and exogenous (e.g. digoxin) inotropic agents. This understanding will lead to better therapy for cardiac disease in the future just as similar knowledge gained in the past has lead to better current therapy for cardiac disease.